This invention relates to methods and apparatus for determining the DC content of an AC waveform and has particular application to determining the DC content in an AC waveform generated by single or multiphase inverters which may be subject to changes in output frequency.
Electronic power generation systems are capable of generating a DC component in an AC output. Such a DC component is often generated in inverters where there are differences in the characteristics of the paired electronic switches generating the AC waveform. This DC component, if excessive, can cause extremely high DC circulating currents in AC loads such as motors and transformers. As a result, the output DC content must be sensed by a system protection circuit to shut down the generating system if internal DC content controls fail.
A commonly assigned application entitled "Detection Of DC Content In An AC Waveform", Ser. No. 225,226, filed Jan 15, 1981 by Shuey et al., now U.S. Pat. No. 4,500,837, discloses methods and apparatus for determining the DC content in an AC waveform and is hereby incorporated by reference. That application discloses a method and apparatus which determines the DC content in an AC waveform by initially measuring and storing the instantaneous value of the AC waveform at a first point in a cycle. A second measurement of the instantaneous value of the waveform is then made at a point spaced from the first point by a fixed time interval which is substantially an exact odd number of waveform half cycles. By measuring the instantaneous value of the waveform at two instants which are separated by an odd number of waveform half cycles, the measured values will be an equal amount above and below the mid-value of the waveform. Averaging the two instantaneous values results in the average value of the waveform and hence the DC content. The disclosed method for determining DC content is independent of the point in the waveform where the first measurement is made. However, the second measurement is always made at a fixed time interval corresponding to an exact odd number of waveform half cycles of the AC waveform at a nominal frequency, following the first point of measurement. If variations in the output frequency of the AC waveform occur, a substantial error can be introduced into the DC content measurement. It is an object of the present invention to provide methods and apparatus for DC content sensing in an AC waveform which may be subject to frequency variations